Electron microscope system and method for evaluating film thickness reduction of resist patterns

ABSTRACT

The invention provides a system for achieving detection and measurement of film thickness reduction of a resist pattern with high throughput which can be applied to part of in-line process management. By taking into consideration the fact that film thickness reduction of the resist pattern leads to some surface roughness of the upper surface of the resist, a film thickness reduction index value is calculated by quantifying the degree of roughness of the part corresponding to the upper surface of the resist on an electron microscope image of the resist pattern which has been used in the conventional line width measurement. The amount of film thickness reduction of the resist pattern is estimated by applying the calculated index value to a database previously made for relating a film thickness reduction index value to an amount of film thickness reduction of the resist pattern.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/354,823, filed Jan. 16, 2009 now U.S. Pat. No. 8,217,348, and whichapplication claims priority from Japanese application serial no.JP2008-040818, filed on Feb. 22, 2008, the contents of which are herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a method for evaluating and managing aresist pattern formed on a wafer in a semiconductor manufacturingprocess, and more particularly to a technique for measuring andevaluating the amount of film thickness reduction of a resist (areduction in height of a resist pattern) using an electron microscopeimage of the resist pattern.

Conventionally, a length-measuring scanning electron microscope (SEM)which is an electron microscope for the purpose of measuring a dimensionof the pattern is widely used as a process management tool in alithography process. The length-measuring SEM enables imaging at highmagnification of a hundred thousand times to three hundred thousandtimes, and thus can measure the dimension of a fine pattern of the orderof several tens of nanometers with an accuracy of 1 nanometer or less.The basic structure of the length-measuring SEM is disclosed inTatsuhiko Higashiki, Ed. “Photolithography II—Measurement and Control—”,ED Research Co., Ltd., pp. 31-41 (2003).

The lithography process involves transferring a circuit pattern onto awafer by exposure and development of a resist, and etching along theresist pattern transferred. The length-measuring SEM is used to measurethe dimension of the transferred resist pattern or the pattern etched.In particular, wiring patterns including a transistor gate wiring aresubjected to strict dimensional control because the width of the patternis strongly related to device performance.

FIG. 2A shows sectional shapes of the patterns before etching, duringetching, after etching in that order from left side thereof. Thelength-measuring SEM measures a resist pattern width W1, or a patternwidth W2 after the etching.

In a conventional lithography process, the resist pattern width W1within the standard allows the pattern width W2 after the etching to berestrained within the standard. Thus, sufficient process management isavailable by monitoring measurement results of the widths W1 and W2.

In order to satisfy the need for microfabrication of patterns, a high NAexposure technique has been developed for obtaining a resolutionrequired for formation of a fine pattern. As a result, a margin for theexposure process becomes small, and a small variation of an exposureparameter, such as a dose amount or a focus position in exposure, leadsto film thickness reduction of the resist pattern, that is, a decreasein height of the resist as compared to the case of the normal exposure.

As shown in FIG. 2B, when a height of a pattern is low (h1<H1) even withthe same pattern width before etching as that shown in FIG. 2A (w1=W1),a pattern width after the etching becomes smaller (w2<W2). During theetching, the thickness of the resist is gradually decreased. When theoriginal pattern thickness is low, the resist pattern almost disappearsduring etching, and then a film of interest for process may be itselfetched as shown in the center drawing in FIG. 2B. As mentioned above,the pattern width after the etching is directly related to the deviceperformance. The condition which may cause the reduction in patternwidth after the etching as shown in FIG. 2B is not appropriate.

In order to introduce the high NA exposure technique, only measurementof the pattern width (W1, w1) is not sufficient in a resist patternstage from a point of view of process management, and also a patternheight (H1, h1) is desired to be measured together with the width.Methods for measuring a pattern height can include measurement using anelectron microscope for observation of a section of a wafer by dividingthe wafer, and measurement using an atomic force microscope (AFM). Theformer needs dividing the wafer, which cannot be apparently applied tothe normal in-line process management. The latter is not appropriate foruse in monitoring the process which needs high throughput.

SUMMARY OF THE INVENTION

The invention provides a system for achieving detection and measurementof film thickness reduction of a resist pattern with high throughputwhich can be applied to part of in-line process management.

That is, the invention has been made by taking into consideration thefollowing. When a resist pattern is formed under an exposure conditiondeviating from the normal exposure condition, the resist pattern hasfilm thickness reduction together with some surface roughness(roughness) of the upper surface of the resist. Thus, the invention isadapted to calculate an index value of film thickness reduction of theresist pattern with respect to a resist pattern formed on the normalexposure condition by quantifying the degree of roughness of a partcorresponding to the upper surface of the resist on an electronmicroscope image of the resist pattern.

The invention not only calculates a film thickness reduction indexvalue, but also estimates the amount of film thickness reduction of aresist pattern actually formed as compared to the resist pattern formedon the normal exposure condition by applying the calculated index valueto a database previously stored for relating a film thickness reductionindex value to an amount of film thickness reduction of a resistpattern.

Further, the invention is adapted to make the database for relating thefilm thickness reduction index value to the amount of film thicknessreduction of the resist pattern by calculating the index values fromelectron microscope images of the resist patterns formed under variousexposure conditions, measuring the heights of the resist patterns by aheight measuring device, and registering the relationship between thefilm thickness reduction index values and the results of measurement ofthe heights.

That is, the invention provides an electron microscope system whichincludes electron beam image obtaining means for obtaining an image of aspecimen having a resist pattern formed on a surface thereof using ascanning electron microscope, quantifying means for quantifying afeature of variations in brightness of the image at a desired area ofthe resist pattern by processing the obtained image, index valuecalculating means for calculating an index value for relating thefeature of variations in brightness of the image quantified by thequantifying means to an amount of reduction from a reference value of afilm thickness of the resist pattern, and display means for displayinginformation about the index value calculated by the index valuecalculating means on a screen.

Further, the invention provides an electron microscope system whichincludes electron beam image obtaining means for obtaining an image of aspecimen having a resist pattern formed on a surface thereof using ascanning electron microscope, quantifying means for quantifying afeature of variations in brightness of the image at a desired area ofthe resist pattern by processing the obtained image, index valuecalculating means for calculating an index value for relating thefeature of variations in brightness of the image quantified by thequantifying means to an amount of reduction from a reference value of afilm thickness of the resist pattern, estimation means for estimatingthe amount of reduction from the reference value of the film thicknessof the resist pattern using the index value calculated by the indexvalue calculating means, and display means for displaying informationabout the amount of reduction from the reference value of the filmthickness of the resist pattern calculated by the estimation means on ascreen.

Further, the invention provides a method for evaluating film thicknessreduction of a resist pattern using an electron microscope system whichincludes the steps of obtaining an image of a specimen having a resistpattern formed on a surface thereof using a scanning electronmicroscope, quantifying a feature of variations in brightness of theimage at a desired area of the resist pattern by processing the obtainedimage, calculating an index value for relating the feature of variationsin brightness of the image quantified to an amount of reduction from areference value of a film thickness of the resist pattern, anddisplaying information about the index value calculated on a screen.

Moreover, the invention provides a method for evaluating film thicknessreduction of a resist pattern using an electron microscope system whichincludes the steps of obtaining an image of a specimen having a resistpattern formed on a surface thereof using a scanning electronmicroscope, quantifying a feature of variations in brightness of theimage at a desired area of the resist pattern by processing the obtainedimage, calculating an index value for relating the feature of variationsin brightness of the image quantified to an amount of reduction from areference value of a film thickness of the resist pattern, estimatingthe amount of reduction from the reference value of the film thicknessof the resist pattern using the calculated index value, and displayinginformation about the amount of reduction from a reference value of afilm thickness of the resist pattern estimated on a screen.

Accordingly, the invention can detect and measure the film thicknessreduction using the image, which has been used by a conventional linewidth measurement method, and thus can monitor the film thicknessreduction, which is a serious process failure, without reducing athroughput of the conventional process management. The invention isapplied to extraction of conditions for the exposure process, that is,optimization of the dose amount and focus position thereby to enablemore effective setting of a process window as compared to the use ofonly the conventional line width measurement result.

These and other objects, features and advantages of the invention willbe apparent from the following more particular description of preferredembodiments of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram showing a configuration of a length-measuring SEMsystem having a function of measuring film thickness reduction accordingto a first embodiment of the invention;

FIG. 1B is a process flowchart of measurement of the film thicknessreduction performed by an arithmetic processor in the first embodiment;

FIG. 1C is a flowchart showing procedures for making a databaseregarding a relationship between a film thickness reduction index valueand an amount of film thickness reduction of a pattern actually formedbased on a resist pattern formed on a normal exposure condition in thefirst embodiment;

FIG. 2A shows pattern sectional shapes of a film of interest for theprocess having a high resist pattern formed thereon before etching,during etching, and after etching;

FIG. 2B shows pattern sectional shapes of a film of interest for theprocess having a low resist pattern formed thereon as compared to thecase shown in FIG. 2A, before etching, during etching, and afteretching;

FIG. 3A is a diagram showing an SEM image of a pattern on a film ofinterest for the process which does not cause the film thicknessreduction after the etching;

FIG. 3B is a diagram showing the SEM image of another pattern on anotherfilm of interest having a reduced thickness after the etching;

FIG. 3C is a diagram showing a signal waveform obtained by adding up SEMimage signals shown in FIG. 3A or 3B in a Y-axis direction;

FIG. 3D is a histogram showing distribution of signal intensities of thesignal waveform shown in FIG. 3C;

FIG. 3E is a diagram showing a signal waveform obtained by adding up SEMimage signals shown in FIG. 3A or 3B in an X-axis direction;

FIG. 4A is a diagram showing SEM images of sections of various patternswith different amounts of film thickness reduction;

FIG. 4B is a diagram showing signal waveforms each obtained by adding upthe SEM image signals corresponding to each sectional pattern in theY-axis direction;

FIG. 4C is a histogram showing distribution of signal intensitiesdetermined from each signal waveform shown in FIG. 4B;

FIG. 4D is a signal waveform obtained by adding up the SEM image signalsshown in FIG. 4A in the X-axis direction;

FIG. 5 shows a GUI screen for registering the relationship between thefilm thickness reduction index value and the amount of film thicknessreduction in the first embodiment;

FIG. 6 shows an output screen of estimation results of the amounts offilm thickness reduction in the first embodiment;

FIG. 7A is a flowchart showing a flow of a process for calculating andoutputting a film thickness reduction index value in a secondembodiment;

FIG. 7B is a graph showing a trend of change in film thickness reductionindex value;

FIG. 8A is a diagram showing a configuration of the length-measuring SEMsystem having a function of measuring film thickness reduction accordingto a third embodiment;

FIGS. 8B and 8C show flowcharts for explaining the first embodiment, inwhich FIG. 8B is a diagram showing a process flowchart of film thicknessreduction measurement performed by an arithmetic processor in the thirdembodiment, and FIG. 8C is a flowchart showing procedures for making adatabase regarding a relationship between the film thickness reductionindex value and the amount of film thickness reduction of a patternactually formed based on a resist pattern formed on a normal exposurecondition in the third embodiment;

FIG. 9 shows a GUI screen for registering the relationship between thefilm thickness reduction index value and the etch bias in the thirdembodiment; and

FIG. 10 shows an output screen of estimation results of the etch bias inthe third embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, a method for measuring film thickness reduction of a resist usingan electron microscope according to the invention will be describedbelow with reference to the accompanying drawings.

(First Embodiment)

The configuration of an SEM system associated with the measurementmethod of film thickness reduction of the resist using the electronmicroscope of the invention, and the entire flow of the method will bedescribed below. Then, each step of the method will be described indetail, and further, the SEM system employing thereon the method will bedescribed.

[SEM System]

FIG. 1A shows a configuration of the length-measuring SEM system havinga function of measuring film thickness reduction according to the firstembodiment. The length-measuring SEM system of this embodiment includesan SEM main unit 10, an image processing and whole controller 109, anarithmetic processor 110, and a database section 113, and is connectedto a data server 116 via a network.

The SEM main unit 10 includes an electron gun 101, an acceleratingelectrode 103 for accelerating an electron beam 102 emitted from theelectron gun 101, a focusing lens 104, and a deflecting lens 105 fordeflecting a track of the electron beam 102. The SEM main unit 10 alsoincludes an objective lens 106 for controlling a position of focus ofthe electron beam 102 such that the focus position on which the electronbeam 102 converges is positioned on a surface of a specimen 107 with apattern formed thereon, and a detector 108 for detecting part ofsecondary electrons generated from the specimen 107 to which theelectron beam 102 is applied. A detection signal of the detector 108 isfed to and processed by the image processing and whole controller 109thereby to obtain an SEM image. The SEM image is processed in use by thearithmetic processor 110 with reference to information stored in thedatabase section 113, and information regarding film thickness reductionis extracted. The result is fed to and stored in the data server 116 viaa communication line.

The specimen 107 is put on a table (not shown). The table (not shown) iscontrolled by the image processing and whole controller 109 such that adesired area on the specimen is positioned at an area for irradiation ofthe electron beam 102.

The arithmetic processor 110 includes a film thickness reduction indexvalue measuring section 111, a database verification section 112, a filmthickness reduction amount estimating section 114, and an input andoutput section 115 with a display screen.

[Entire Flow]

FIG. 1B is an entire flowchart of measurement of film thicknessreduction performed by the arithmetic processor 110.

(Step S1): A resist pattern is imaged by the SEM 10, and a signalobtained by the imaging is processed by the image processing and wholecontroller 109 to obtain an SEM image of the specimen.

(Step S2): An index value of film thickness reduction is calculated fromthe obtained image by a method to be described later.

(Step S3): The film thickness reduction index value calculated isverified against a database 20 which registers a relationship betweenfilm thickness reduction index values and amounts of film thicknessreduction of patterns actually formed based on a resist pattern formedunder a normal exposure condition stored in the database section 113. Amethod for making the database 20 will be described later using FIG. 1C.(Step S4): The amount of film thickness reduction of the patternactually formed according to the resist pattern formed under the normalexposure condition is estimated based on the result of verification. Theverification result is displayed on a display screen of the input andoutput section 115.

The entire of the film thickness reduction measurement has beendescribed. Each step of the flow will be described below in detail.

[Calculation of Film Thickness Reduction Index Value]

Step S2 shown in FIG. 1B will be described below in detail. The filmthickness reduction of the pattern actually formed based on the resistpattern formed under a normal exposure condition is caused by applyingexcessive light to a resist pattern portion to which the light shouldnot be applied inherently, in particular, an upper portion of the resistpattern. The irradiation of the upper portion of the resist pattern withthe excessive light may be caused by focus displacement in exposure, orby an increase in dose amount.

Resist material commonly used has such properties that the more theamount of irradiated light, the larger the roughness of a surface(surface roughness). The film thickness reduction is caused byirradiating the resist pattern upper portion with the excessive light,which inevitably leads to roughness of the surface of the upper portionof the resist pattern.

In the invention, this mechanism is used to quantify a change inroughness of the surface of the pattern upper portion on the SEM imagetogether with the film thickness reduction, and the quantified change isused as a film thickness reduction index value.

FIG. 3A shows an SEM image without film thickness reduction, and FIG. 3Bshows an SEM image having film thickness reduction. In the SEM image, aflat portion is detected to be dark, and an inclined portion is detectedto be bright by an inclined angle effect. The edge of the surface withroughness is detected to be bright in a streaky manner. As shown in FIG.3A, a portion enclosed by a dotted line corresponds to the upper surfaceof the resist pattern. The reason for irregular bright areas shown inFIG. 3B is due to existence of the roughness as compared to the caseshown in FIG. 3A. Now, methods for quantifying features on the imageinclude the following examples.

(A): A projected waveform on a Y axis of an area (an area enclosed bythe dotted line shown in FIGS. 3A and 3B) corresponding to an upperportion of a pattern in an SEM image is generated (in FIG. 3C, alongitudinal axis SE intensity indicates the brightness of the SEMimage, and a lateral axis Y indicates the position). The brightnessvariation of the projected waveform is represented by the standarddeviation σ to be used as a roughness index value, that is, a resistfilm thickness reduction index value. As the roughness becomes larger,variations in brightness are increased, resulting in large standarddeviation 6. FIG. 4B shows the projected waveforms on the Y axis forvarious amounts of film thickness reduction (as the reference numberincreases from (i) to (v), the amount of film thickness reductionbecomes larger). As shown in the figure, as the amount of film thicknessreduction becomes large, the index value takes a large one. According tothis method, since a certain zone is projected to enable decreasing theinfluence of high frequency noise specific to the SEM by an averagingeffect, the brightness variation due to the surface roughness can bedetected with good sensitivity. The averaging processing is performed inthe direction of width of a pattern line, which can reduce an influenceof change in brightness due to a change in shape of the pattern otherthan the film thickness reduction. An area of the pattern in thelongitudinal direction for generating the projected waveform is madelonger, which can improve the high accuracy of the film thicknessreduction index value. In graphs shown in (i) to (v) in FIG. 4B, thelongitudinal axis and the lateral axis are the same as those shown inFIG. 3C, and a description thereof is omitted below.(B): A brightness histogram of an area corresponding to the patternupper portion in the SEM image (enclosed by the dotted line shown inFIGS. 3A and 3B) is made (see FIG. 3D), and then the histogram isapplied to a normal distribution. At that time, the σ value or thecenter of the normal distribution is used as a resist film thicknessreduction index value. Specifically, the histogram is made bydetermining an appearance frequency for each brightness on thelongitudinal axis from the projected waveform onto the Y axis determinedby the above-mentioned method (A). The concept of this method is thesame as that of the above method (A). This method involves qualifying byuse of the feature that as the roughness becomes larger, the brightnessvariation is increased averagely toward the brighter level. FIG. 4Cshows a histogram of each of various amounts of film thickness reduction(as the reference number increases from (i) to (v), the amount of filmthickness reduction becomes large). Also, as the amount of filmthickness reduction becomes larger, the index value takes a larger one.In the graphs shown in (i) to (v) in FIG. 4C, the longitudinal axis andthe lateral axis are the same as those shown in FIG. 3D, and adescription thereof is omitted below.(C): A projected waveform of the SEM image on the X axis is generated,and the minimum value of the center of the waveform (indicated by “min”shown in FIG. 3E) is set as the film thickness reduction index value.The edge having some surface roughness is detected to be bright in thestreaky manner with increasing roughness. Thus, this method involvesqualifying by use of the feature that as the roughness becomes larger,the brightness is increased averagely. FIG. 4D shows a projectedwaveform on the X axis of each of various amounts of film thicknessreduction (as the reference number increases from (i) to (v), the amountof film thickness reduction becomes large). Also, as the amount of filmthickness reduction becomes large, the index value takes a large one.This method advantageously has an excellent effect of reducing SEM noiseby averaging over a wide area. This index value is apt to be influencedby a change in shape other than the film thickness roughness, inparticular, together with the change in shape of the pattern topportion. Thus, the index value is effective for an object whose changein shape other than the film thickness reduction is small. In the graphsshown in (i) to (v) in FIG. 4D, the longitudinal axis and the lateralaxis are the same as those shown in FIG. 3E, and a description thereofis omitted below.(D): A method for texture analysis can be applied as disclosed in MikioTakagi and Haruhisa Shimoda, Ed. “Image Analysis handbook”, Universityof Tokyo Press, Function I, chapter 2 (1991). That is, a power spectrumof an area corresponding to the upper portion in the SEM image isdetermined, and the roughness of a texture is quantified from frequencyproperties. Alternatively, another method for quantifying the featuresof the image can involve combining some of the above various methodstogether. A further method can involve weighting and adding up variousindex values in use.[Method for Making Database 20]

FIG. 1C shows a flowchart for making the database 20 stored in thedatabase section 113.

(Step S11): Samples with resist patterns formed under various exposureconditions are prepared for making the database 20. For example, a focusexposure matrix (FEM) wafer may preferably be used. The FEM wafer hasthe focus position and dose amount of one wafer changed as exposureconditions within a range in which these conditions can be varied.(Step S12): SEM images of the resist pattern samples prepared and formedunder the various exposure conditions are obtained, and then filmthickness reduction index values are calculated by any one of themethods (A) to (D) described in the above paragraph “Calculation of FilmThickness Reduction Index Value”.(Step S13): The height of the resist pattern of each same sample ismeasured by height measuring means, such as an atom force microscope(AFM), a scatterometry (lightwave scattering measurement), orobservation of a pattern section using the SEM, thereby to determine theamount of film thickness reduction of the resist. Instead of actuallymeasuring the height of the pattern, the height determined bycalculating the sectional shape of the resist pattern under each of thevarious exposure conditions by an exposure simulation may be used.(Step S14): The relationship between the thus-obtained amount of filmthickness reduction and the film thickness reduction index value isshown in the graph 21. As mentioned above, the larger the amount of filmthickness reduction, the larger the film thickness reduction indexvalue. In response to the result, an approximate function is calculatedas the relationship between the amount of film thickness reduction andthe film thickness reduction index value. At the same time, variationsin amount of film thickness reduction respective to the film thicknessreduction index value may be calculated. A look-up table format may beused in place of the approximate function.(Step S15): The approximate function calculated as the relationshipbetween the amount of film thickness reduction and the film thicknessreduction index value is stored in the database. The relationship ofvariations in amount of the film thickness reduction with respect to thecalculated film thickness reduction index value is stored in thedatabase.(Verification Against Database and Estimation of Amount of FilmThickness Reduction)

Steps S3 and S4 shown in FIG. 1B will be described below in detail. Instep S3 shown in FIG. 1B, an absolute value of the amount of filmthickness reduction is estimated by substituting the index valuecalculated in step S2 into the approximate function calculated as therelationship between the amount of film thickness reduction and the filmthickness reduction index value by the above database 20, or byreferring to the look-up table based on the relationship between theamount of film thickness reduction and the film thickness reductionindex value. The use of the index value calculated in step S2 can graspan error of the above estimation result of the amount of film thicknessreduction from the relationship of variations in amount of filmthickness reduction with respect to the index value which relationshipis stored in the database 20.

[GUI]

FIG. 5 shows an example of a graphic user interface (GUI) screen 500,which is one example of an input and output screen of the input andoutput section 115 of the SEM system in this embodiment. The GUI screenincludes an input screen required to form and update the database 20 ofthe database section 113 for previously storing the relationship betweenthe film thickness reduction amount and the film thickness reductionindex value.

The GUI screen 500 includes an input portion 501 for inputting a name ofthe database 20 to be newly registered or updated. The GUI screen 500displays information selected from the database 20 according to the nameinput in the form of a data list 506 and a graph 507. The GUI screen 500also includes a portion 502 for selecting a measurement recipe forobtaining a film thickness reduction index value for registration, andan image display area 504 for displaying an image 503 registered in therecipe selected. The GUI screen 500 further includes a portion 505 forselecting an area on the SEM image used for calculation of the filmthickness reduction index value on the image 503 displayed.

After inputting these items, a cursor (not shown) is moved to a button508 for execution of the recipe and then a mouse (not shown) is clicked,whereby calculation of a film thickness reduction index value isperformed based on the set conditions. Data about the film thicknessreduction index value calculated is automatically updated, and then thedata list 506 is also corrected. Further, data about the amount of filmthickness reduction obtained by another device or means can be inputinto the data list 506 displayed on the screen 500. In order to storethe input data in the database 20, an updating button 509 may bepreferably clicked. This operation is performed to make the relationshipbetween the amount of film thickness reduction and the film thicknessreduction index value in the form of function, which is registered inthe database 20.

The input and output section 115 of the SEM system further displays aGUI screen 600 which includes a setting section for verification againstthe information in the database 20 and for estimation of the amount offilm thickness reduction, and an output section for displaying theresult. FIG. 6 shows an example of the GUI screen 600.

In setting a measurement condition, the GUI screen 600 includes aportion 601 for selecting the database 20 for calculation of the filmthickness reduction amount for reference, and a portion 602 fordisplaying an area for use in the film thickness reduction index valueon the SEM image automatically obtained by the selection based on theinformation in the database. The GUI screen 600 further includes aportion (film thickness reduction evaluation image selector) 603 forselecting an image for evaluating the amount of film thickness reductionfrom the sets of images obtained, and an exertion button 604 forinstructing calculation of the film thickness reduction index value forthe selected image and for estimation of the amount of film thicknessreduction. The GUI screen 600 also includes a portion 606 for displayingthe selected image 605.

The GUI screen 600 further includes a portion (a recipe selector forevaluation of film thickness reduction) 607 for selecting a measurementrecipe, and an execution button 608 for calculation of the filmthickness reduction index value and for estimation of the amount of filmthickness reduction based on the recipe selected. The cursor (not shown)is moved to the execution button 608 on the screen 600 and then themouse (not shown) is clicked, whereby calculation of a film thicknessreduction index value and estimation of the amount of film thicknessreduction are performed based on the set conditions. The result isdisplayed in the form of the graph 610 and the data list 609 togetherwith estimated errors.

As mentioned above, the invention can detect and measure the filmthickness reduction using the image, which has been used by aconventional line width measurement method, and thus can monitor thefilm thickness reduction without reducing a throughput of theconventional process management. The invention may be applied toextraction of conditions for the exposure process, that is, optimizationof the dose amount and focus position in exposure. The condition forpreventing the occurrence of film thickness reduction is incorporated,which can set a more effective process window as compared to the use ofonly the conventional line width.

(Second Embodiment)

A second embodiment of the invention is shown in FIG. 7. In the firstembodiment, the film thickness reduction index value is calculated fromthe SEM image (in step S2), and then is verified against the database(in step S3), whereby the absolute value of the film thickness reductionamount is estimated (in step S4), and the film thickness reduction indexvalue and the amount of film thickness reduction are output. However, inthe second embodiment, only the calculation of the film thicknessreduction index value from the SEM image is performed (in step S2)without estimating the amount of film thickness reduction in steps S3and S4, so that only information regarding the film thickness reductionindex value is output (in step S6). A method for calculating the filmthickness reduction index value from the SEM image in this embodiment(in step S2) is the same as that described in the first embodiment.

GUI screens of the input and output section of this embodiment arebasically the same as those described in FIGS. 5 and 6 except that thecolumn for the amount of film thickness reduction provided in the datalist 506 shown in FIG. 5 is not provided, and that the columns for theestimation result of the film thickness reduction amount and for theestimated error provided in the data list 609 shown in FIG. 6 are notprovided. FIG. 7B shows other examples of outputs in this embodiment.

According to this embodiment, the index value represents a relativechange in amount of film thickness reduction. As shown in FIG. 7B, thetrend of the index value is monitored, whereby the process variation canbe monitored.

(Third Embodiment)

A third embodiment of the invention is shown in FIG. 8. Although thefirst and second embodiments have described the method for measuring theresist pattern formed on the wafer, this embodiment has an object todetect a decrease in line width of a circuit pattern formed on a wafertogether with the film thickness reduction of a resist pattern in thecase of etching using the resist pattern as a mask.

This embodiment employs a database 30 which registers a relationshipbetween a resist line width and an etching bias, that is, between a linewidth of a resist pattern and a line width of the pattern etched,instead of the database for registering the relationship between theamount of film thickness reduction and the film thickness reductionindex value in the first embodiment thereby to estimate an etching biasfrom the result of measurement of the film thickness reduction indexvalue (in steps S23 and S24).

Now, a method for making the database 30, and an SEM system employingthe method will be described below.

[SEM System]

FIG. 8A shows the configuration of a length-measuring SEM systemincluding a function of measuring the film thickness reduction in thisembodiment. The length-measuring SEM system of this embodiment includesan SEM main unit 80, an image processing and whole controller 81, anarithmetic processor 82, and a database section 83. The SEM system isconnected to a data server 84 via a network. The configurations of theSEM main unit 80 and the image processing and whole controller 81 arethe same as those of the first embodiment described with reference toFIG. 1A.

In this embodiment, the SEM image output from the image processing andwhole controller 81 is processed in use by the arithmetic processor 82with reference to information stored in the database section 83, andinformation regarding an etch bias is extracted. The result is fed toand stored in the data server 116 via a communication line.

The arithmetic processor 82 includes a film thickness reduction indexvalue measuring section 821, a database verification section 822, anetch bias estimation section 823, and an input and output section 824having a display screen.

[Entire Flow]

FIG. 8B shows an entire flowchart of estimation of an etch biasperformed by the arithmetic processor 82.

(Step S21): A resist pattern is imaged by the SEM 80, and a signalobtained by the imaging is processed by the image processing and wholecontroller 81 to obtain an SEM image of a specimen.

(Step S22): A film thickness reduction index value is calculated fromthe obtained image in the same way as that in step S2 of the firstembodiment.

(Step S23): The calculated index value is verified against the database30 stored in the database section 83 for registering the relationshipbetween the line width of the resist pattern and the line width of thepattern etched. A method for making the database 30 will be describedlater using FIG. 8C.(Step S24): An etch bias is estimated based on the result ofverification.(Step S25): The estimated result is displayed on the display screen ofthe input and output section 824, and is then fed to and stored in thedata server 84 via a communication line.

The entire flow of estimation of the etch bias in this embodiment hasbeen described above. The details of the main flow will be describedbelow.

[Method for Making Database]

FIG. 8C shows the flow for making the database 30 for registering therelationship between the line width of the resist pattern and the linewidth of the pattern etched in this embodiment.

(Step S31): Samples with resist patterns formed under various exposureconditions are prepared in the same way as that described in step S11 ofthe first embodiment.

(Step S32): The film thickness reduction index value is calculated, andthe line width of the pattern is measured in the same way as that instep S12 of the first embodiment.

(Step S33): The height of the resist pattern is measured in the same wayas that described in step S13 of the first embodiment thereby todetermine the amount of film thickness reduction of the resist.

(Step S34): A wafer with the resist pattern formed thereon is subjectedto etching.

(Step S35): The line width of the pattern formed on the wafer by theetching is measured. The wafer used in this step may be the same as thatused in steps S32 and S33, or alternatively, may be another waferobtained by the same process. Data about the measured line width of thepattern formed on the wafer etched is stored in relation to data aboutthe dimension of the resist pattern measured in step S32, and the heightof the resist pattern measured in step S33.(Step S36): A relationship between the amount of film thicknessreduction of the resist determined in step S33 and an etching bias whichis a difference between the line width of the resist pattern determinedin step S32 and the line width of the etched pattern determined in stepS35 is determined. One example of the relationship determined isrepresented in a graph 31. Based on the result, an approximate functionis calculated, and the relationship between the etching bias and thefilm thickness reduction index value is determined(Step S37): A relationship between the etching bias determined and thefilm thickness reduction index value is stored in the database 30. Atthe same time, the film thickness reduction index value at which theetching bias increases is stored as a threshold, and further, variationsin etching bias with respect to the film thickness reduction index valueis calculated and then stored in the database 30.[Estimation of Etching Bias]

In step S24 of estimating an etching bias as shown in FIG. 8B, the filmthickness reduction index value calculated in step S22 is substitutedinto the approximate function on the database 30 calculated by the abovestep S36 thereby to estimate an etching bias. Alternatively, it isdetermined whether or not the etching bias exceeds the threshold.

[GUI]

FIG. 9 shows an example of the GUI screen 900 as one example of theinput and output screen of the input and output section 84 of the SEMsystem according to this embodiment. The GUI screen is the same as oneobtained by replacing the amount of film thickness reduction by the etchbias on the GUI screen shown in FIG. 5 of the first embodiment.

More specifically, the GUI screen 900 includes an input portion 901 forinputting a name of the database 30 to be newly registered or updated.The GUI screen 900 displays information selected from the database 30according to the name input in the form of a data list 906 and a graph907. The data list 906 of this embodiment displays an etching bias valuein addition to the film thickness reduction index value of the resistpattern, in place of the column for the film thickness reduction amountat the data list 506 of the first embodiment shown in FIG. 5. Theetching bias value is a difference between a resist CD value (adimension value of the resist pattern determined from the SEM image) andan etch CD value (a dimension value of the pattern formed on the waferafter etching which value is determined from the SEM image), and betweena line width of the resist pattern (resist CD value) and a line width ofthe pattern etched (etch CD value). The graph 907 displayed in thisembodiment differs from the case of the first embodiment shown in FIG. 5in that the relationship between the etch bias and the film thicknessreduction index value is displayed.

On the other hand, the screen 900 includes a portion 902 for selecting ameasurement recipe for obtaining a film thickness reduction index valuefor registration, an image display area 904 for displaying an image 903registered in the selected recipe, and a portion 905 for selecting anarea on the SEM image for use in calculation of the film thicknessreduction index value on the displayed image 903. The screen 900 alsoincludes a button 908 for executing the recipe after inputting theseitems, and a data list 906 adapted to be corrected by calculating thefilm thickness reduction index value based on the above set conditions,and automatically updating data, like the first embodiment. Further,data about the amount of film thickness reduction obtained by anotherdevice or means can be input to the data list 906 displayed on thescreen 900. The storage of the input data in the database 30 onlyrequires clicking of an updating button 909. This operation makes therelationship between the amount of film thickness reduction and the filmthickness reduction index value in the form of function, which isregistered in the database 30.

The input and output section 824 of the SEM system displays a GUI screen1000 including a setting section for estimating the amount of filmthickness reduction by verification against the information of thedatabase 30, and an output section for displaying the result. The GUIscreen 1000 has a similar structure to that of the GUI screen 600 shownin FIG. 6 as described in the first embodiment.

Like the first embodiment described above, in setting a measurementcondition, the GUI screen 1000 includes a portion 1001 for selecting thedatabase 30 for calculation of the film thickness reduction amount forreference, and a portion 1002 for displaying an area for use in the filmthickness reduction index value on the SEM image automatically obtainedby the selection based on the information of the database. Further, theGUI screen 1000 also includes a portion (film thickness reductionevaluation image selector) 1003 for selecting an image for evaluatingthe amount of film thickness reduction from the sets of images obtained,and an exertion button 1004 for instructing calculation of the filmthickness reduction index value and for estimation of the amount of filmthickness reduction for the selected image. The GUI screen 1000 furtherincludes a portion 1006 for displaying an image 1005 selected.

The GUI screen 1000 further includes a portion (recipe selector forevaluation of film thickness reduction) 1007 for selecting a measurementrecipe, and an execution button 1008 for calculation of the filmthickness reduction index value and for estimation of the amount of filmthickness reduction based on the recipe selected. The cursor (not shown)is moved to the execution button 1008 on the screen 1000 and then themouse (not shown) is clicked thereby to perform calculation of a filmthickness reduction index value and of an etch bias, estimation of anerror of the amount of film thickness reduction, and determination ofNG, based on the set conditions. The results are displayed in the datalist 1009, and the relationship between the etch bias and the filmthickness reduction index value is displayed as the graph 1010.

According to this embodiment, as shown in the graph 31 of FIG. 8, therelationship between the amount of film thickness reduction and thedimension of the pattern etched is not linear. When the amount of filmthickness reduction exceeds a certain value, the etch bias quicklyincreases. This embodiment can determine whether or not the filmthickness reduction caused is at a problematic level from a viewpoint ofdevice characteristics.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

What is claimed is:
 1. An electron microscope system, comprising:electron beam image obtaining means for obtaining an image of a specimenhaving a resist pattern formed on a surface thereof using a scanningelectron microscope; quantifying means for quantifying a feature ofvariations in brightness of the image at a desired area of the resistpattern by processing the obtained image; index value calculating meansfor calculating an index value for relating the feature of variations inbrightness of the image quantified by the quantifying means to an amountof reduction from a reference value of a film thickness of the resistpattern; and estimation means for estimating the amount of reductionfrom the reference value of the film thickness of the resist patternusing the index value calculated by the index value calculating means,wherein the index value is correlated with changes in roughness of asurface of the resist pattern at the desired area, and furthercomprising means for verifying the calculated index value against adatabase which registers a relationship between film thickness reductionindex values and amounts of film thickness reduction of resist patternsor a relationship between an etching bias and film thickness reductionindex values.
 2. The electron microscope system according to claim 1,wherein the index value calculating means calculates the index value forrelating the feature of variations in brightness of the image quantifiedby the quantifying means to the amount of reduction from the referencevalue which is the film thickness of the resist pattern formed by beingexposed under a normal exposure condition.
 3. The electron microscopesystem according to claim 1, wherein the estimation means estimates anamount of film thickness reduction of the resist pattern by applying theindex value calculated by the index value calculation means to adatabase previously made and stored for relating an index value for aresist pattern having a reference film thickness to an amount of filmthickness reduction of the resist pattern.
 4. The electron microscopesystem according to claim 1, wherein the index value having acorrelation with the amount of film thickness reduction calculated fromthe electron microscope image is a quantified degree of roughness of thedesired area of the resist pattern caused by a phenomenon of the filmthickness of the resist pattern with respect to the reference thicknessof the resist pattern as a reference.
 5. The electron microscope systemaccording to claim 1, wherein the index value having the correlationwith the amount of film thickness reduction calculated from the electronmicroscope image is a quantified average brightness of the desired areaof the resist pattern.
 6. The electron microscope system according toclaim 1, wherein the index value having the correlation with the amountof film thickness reduction calculated by the electron microscope imageis a quantified distribution of the brightness of the desired area ofthe resist pattern.
 7. The electron microscope system according to claim1, wherein the film thickness reduction index values comprise standarddeviations of a brightness variation of projected waveform of the imageof a specimen.
 8. The electron microscope system according to claim 1,wherein the film thickness reduction index values comprise σ values orcenters of a normal distribution applied to a brightness histogram ofthe image of a specimen.
 9. The electron microscope system according toclaim 1, wherein the film thickness reduction index values compriseminimum values of a center of a waveform of the image of a specimen. 10.The electron microscope system according to claim 1, wherein theroughness of the surface of the resist pattern is determined by atexture analysis in which a power spectrum of an area corresponding toan upper position in image is determined, and the roughness of a textureis quantified from frequency properties of the power spectrum.
 11. Amethod for evaluating film thickness reduction of a resist pattern usingan electron microscope system, comprising: obtaining an image of aspecimen having a resist pattern formed on a surface thereof using ascanning electron microscope; quantifying a feature of variations inbrightness of the image at a desired area of the resist pattern byprocessing the obtained image; calculating an index value for relatingthe feature of variations in brightness of the image quantified by thequantifying step to an amount of reduction from a reference value of afilm thickness of the resist pattern; and estimating the amount ofreduction from the reference value of the film thickness of the resistpattern using the index value calculated by the index value calculatingstep, wherein the index value is correlated with changes in roughness ofa surface of the resist pattern at the desired area, and furthercomprising verifying the calculated index value against a database whichregisters a relationship between film thickness reduction index valuesand amounts of film thickness reduction of resist patterns or arelationship between an etching bias and film thickness reduction indexvalues.
 12. The method according to claim 11, wherein the index valuecalculating step calculates the index value for relating the feature ofvariations in brightness of the image quantified by the quantifying stepto the amount of reduction from the reference value which is the filmthickness of the resist pattern formed by being exposed under a normalexposure condition.
 13. The method according to claim 11, wherein theestimation step estimates an amount of film thickness reduction of theresist pattern by applying the index value calculated by the index valuecalculation step to a database previously made and stored for relatingan index value for a resist pattern having a reference film thickness toan amount of film thickness reduction of the resist pattern.
 14. Themethod according to claim 11, wherein the index value having acorrelation with the amount of film thickness reduction calculated fromthe electron microscope image is a quantified degree of roughness of thedesired area of the resist pattern caused by a phenomenon of the filmthickness of the resist pattern with respect to the reference thicknessof the resist pattern as a reference.
 15. The method according to claim11, wherein the index value having the correlation with the amount offilm thickness reduction calculated from the electron microscope imageis a quantified average brightness of the desired area of the resistpattern.
 16. The method according to claim 11, wherein the index valuehaving the correlation with the amount of film thickness reductioncalculated by the electron microscope image is a quantified distributionof the brightness of the desired area of the resist pattern.
 17. Themethod according to claim 11, wherein the film thickness reduction indexvalues comprise standard deviations of a brightness variation ofprojected waveform of the image of a specimen.
 18. The method accordingto claim 11, wherein the film thickness reduction index values compriseσ values or centers of a normal distribution applied to a brightnesshistogram of the image of a specimen.
 19. The method according to claim11, wherein the film thickness reduction index values comprise minimumvalues of a center of a waveform of the image of a specimen.
 20. Themethod according to claim 11, wherein the roughness of the surface ofthe resist pattern is determined by a texture analysis in which a powerspectrum of an area corresponding to an upper position in image isdetermined, and the roughness of a texture is quantified from frequencyproperties of the power spectrum.